The invention relates to a casting furnace for producing castings which are directionally solidified in monocrystalline and polycrystalline form.
Such a casting furnace can be used to produce components which are of complicated design and can be exposed to high thermal and mechanical loads, for example guide vanes and rotor blades of gas turbines. Depending on the process conditions, the directionally solidified casting may be formed as a single crystal (SX) or may be in polycrystalline form from preferentially oriented columnar crystals (directionally solidified, DS). It is of particular importance for the directional solidification to take place-under conditions in which there is considerable heat exchange between a cooled part of a casting mold holding molten starting material and the still molten starting material. It is then possible for a zone of directionally solidified material to form with a solidification front which, as heat continues to be withdrawn, migrates through the casting mold so as to form the directionally solidified casting.
Document EP-A1-749,790 has disclosed such a process and a device for producing a directionally solidified casting. The device comprises a vacuum chamber which contains an upper heating chamber and a lower cooling chamber. The two chambers are separated by a baffle. The vacuum chamber accommodates a casting mold which is filled with a molten material. To produce parts which can be subjected to thermal and mechanical loads, as in the case of guide vanes and rotor blades of gas turbines, a nickel base superalloy, for example, is used. In the center of the baffle, there is an opening through which the casting mold is slowly moved from the heating chamber into the cooling chamber during the process, so that the casting is directionally solidified from the bottom upward. The downward movement is brought about by means of a drive rod on which the casting mold is mounted. The base of the casting mold is of water-cooled design. Beneath the baffle there are means for generating and guiding a gas flow. Through the flow of gas next to the lower cooling chamber, these means provide additional cooling and thus a greater temperature gradient at the solidification at the front.
A similar process which, in addition to heating and cooling chambers, operates with additional gas cooling, is known, for example, from U.S. Pat. No. 3,690,367.
A further process for producing directionally solidified castings with heating and cooling chambers is also described, for example, in document U.S. Pat. No. 3,532,155.
A further process for producing a directionally solidified casting is known from document U.S. Pat. No. 3,763,926. In this process, a casting mold which has been filled with a molten alloy is immersed continuously into a bath which has been heated to approx. 260xc2x0 C. This results in particularly rapid dissipation of heat from the casting mold. This and other similar processes are known as LMC (liquid metal cooling).
In all the abovementioned process variants, the upper heating chamber comprises one or more heater elements which surround the casting mold located therein from the outside and are usually of cylindrical form, and a thermal insulation which covers the heating chamber at the top. To achieve improved productivity and a uniform quality of casting, particularly for the production of turbine blades as many casting pieces as possible are arranged symmetrically in a casting mold on an imaginary circle or in a similar manner.
A significant drawback of the abovementioned processes is that, owing to the externally arranged heater, in the heating chamber heat is preferentially introduced into those surfaces of the casting mold which face outward. Particularly in the case of vacuum furnaces, the heat transfer takes place only by means of radiation. If a plurality of castings are arranged in the form of a circle or the like in a casting mold, the casting mold shadows some of the thermal radiation coming from the heater, so that those surfaces of the casting mold which face inward into the center of the heating chamber are cooler than the surfaces which face outward toward the heater element. This results in a sloping solidification front in the casting pieces, i.e. the solidification front deviates significantly from the horizontal position which is desired during the solidification process. In directionally solidified, polycrystalline casting pieces, it is a drawback for the grain boundaries to be sloping, resulting in undesirable coarsening of the grains. In the case of single crystal and directionally solidified polycrystalline casting pieces, this sloping position of the solidification front may result in undesirable flawed grains. In the case of many directionally solidifying alloys, a sloping position of the solidification front also promotes the formation of undesirable flaws, known as freckles, i.e. a series of small flawed grains arranged in the vertical direction.
The object of the invention is to eliminate the described drawback and to provide a casting furnace for producing directionally solidified castings which avoids the sloping position which occurs at the solidification front.
According to the invention there is provided a casting furnace for producing castings which are directionally solidified in monocrystalline and polycrystalline form, comprising an upper heating chamber with a heating chamber wall, the chamber contains at least one heater element, a furnace cover, a lower cooling chamber, a casting mold with casting pieces, a conveyor device for the casting mold, and an internal heater which contains at least one heater element and is arranged in the middle area of the upper heating chamber centrally between the casting pieces.
This internal heater heats the cooler surfaces of the mold, facing inward into the center of the heating chamber, so that the solidification front runs substantially horizontally through the casting pieces. As a guideline, the internal heater, which may comprise one or more individual heaters, should be at the same temperature as the outer heater element(s) at a similar level in the heating chamber.
Advantageously, the internal heater is arranged mechanically on the casting furnace cover. The lower area is thermally insulated with respect to the lower cooling chamber, in order to avoid heat loss to this chamber and to produce a greater temperature gradient at the solidification front. To provide the insulation, an internal baffle may be arranged in the middle area between the upper heating chamber and the lower cooling chamber. The casting furnace according to the invention makes it possible to achieve increased productivity and a more uniform quality of casting, since a larger number of casting pieces can be arranged in the casting furnace without suffering a loss of quality such as that which is known from the prior art. The internal heater may be designed in the form of a rod or a hollow cylinder. In the case of a hollow cylindrical heater, the casting mold is filled from the top through the heater with the aid of a funnel, in which case the inner surface of the heater may be thermally insulated.